1. Field of the Invention
The present invention relates to a nitride-based semiconductor element and a method of forming a nitride-based semiconductor, and more specifically, it relates to a nitride-based semiconductor element having a group III-V nitride-based semiconductor (hereinafter referred to as a nitride-based semiconductor) such as GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride) or TlN (thallium nitride) or a mixed crystal thereof and a compound semiconductor layer consisting of a group III-V nitride-based semiconductor such as a mixed crystal containing at least one element of As, P and Sb and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
A semiconductor element utilizing a GaN-based compound semiconductor is actively developed as a semiconductor element applied to a semiconductor light-emitting device such as a light-emitting diode or an electronic element such as a transistor. In order to fabricate such a GaN-based semiconductor element, a GaN-based semiconductor layer is epitaxially grown on a substrate consisting of sapphire, SiC, Si, GaAs or the like, since a substrate consisting of GaN is hard to fabricate.
However, the substrate of sapphire or the like and GaN have different lattice constants. Therefore, threading dislocations (lattice defects) vertically extending from the substrate are present in the GaN-based semiconductor layer grown on the substrate of sapphire or the like. The dislocation density of these lattice defects is about 109 cmxe2x88x922. Such dislocations in the GaN-based semiconductor layer deteriorate the element characteristics of the semiconductor element and reduce the reliability thereof.
In order to reduce the number of dislocations in the aforementioned GaN-based semiconductor layer, therefore, epitaxial lateral overgrowth (ELO) is proposed in general. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai (1998) pp. 53 to 58 and pp. 210 to 215, for example.
In a method of forming a GaN-based semiconductor layer employing the aforementioned epitaxial lateral overgrowth, selective growth masks are first formed on prescribed regions of an underlayer. A GaN-based semiconductor layer is epitaxially laterally overgrown from exposed portions of the underlayer. In this case, facets of the GaN-based semiconductor having a triangular section are grown upward from the exposed portions of the underlayer, and thereafter epitaxially laterally grown on the selective growth masks. Thus, the facets coalesce with each other on the selective growth masks, to form a continuous film. Thus, a flat GaN-based semiconductor layer is formed on the underlayer and the selective growth masks. Crystal defects of the underlayer only partially propagate to the GaN-based semiconductor layer obtained by such epitaxial lateral overgrowth, and hence the dislocation density can be reduced to about 107 cmxe2x88x922.
Further, a method of reducing the number of dislocations in a GaN-based semiconductor layer through a dislocation loop effect of quantum dots is developed in general. This method is disclosed in Jpn. J. Appl. Phys. Vol. 39 (2000), L831-834, for example. In the aforementioned conventional method utilizing the dislocation loop effect of the quantum dots, dislocations of an underlayer are trapped in the quantum dots in a looped manner, to be only partially propagated to a GaN-based semiconductor layer. Thus, a GaN-based semiconductor layer having a small number of dislocations can be formed.
However, the aforementioned methods of reducing the number of dislocations in the nitride-based semiconductor by epitaxial lateral overgrowth and the quantum dots respectively have the following problems:
In the aforementioned method of reducing the dislocation density of the nitride-based semiconductor through the epitaxial lateral overgrowth, laterally grown layers (facets) for forming the nitride-based semiconductor layer coalesce (bond) with each other on central portions of the selective growth masks, and hence portions having relatively high dislocation density are disadvantageously formed above the central portions of the selective growth masks (above bonding portions of the facets). Further, portions having relatively high dislocation density are disadvantageously formed above the central portions of openings of the selective growth masks (around the tops of the facets) due to relatively high dislocation density around the tops of the facets.
In order to solve the aforementioned problems, a method of repeating epitaxial lateral overgrowth thereby reducing the number of dislocations is proposed in Jpn. J. Appl. Phys. Vol. 39 (2000) L647-650, for example. In this conventional proposed method, a first GaN-based semiconductor layer is formed on selective growth masks provided on an underlayer by first epitaxial lateral overgrowth, followed by formation of selective growth mask layers on the first GaN-based semiconductor layer. A second GaN-based semiconductor layer is formed on the first GaN-based semiconductor layer by second selective epitaxial lateral overgrowth, so that the second GaN-based semiconductor layer is further reduced in dislocation density as compared with the first GaN-based semiconductor layer. Such epitaxial lateral overgrowth is so repeated that a GaN-based semiconductor layer having a smaller number of dislocations can be formed.
In the aforementioned conventional proposed method, however, epitaxial lateral overgrowth must be repeated and hence the step of forming the GaN-based semiconductor layer is disadvantageously complicated.
In the aforementioned conventional proposed method, further, a plurality of GaN-based semiconductor layers must be formed by repeating epitaxial lateral overgrowth. Therefore, the thickness of the wafer is so increased that the wafer is disadvantageously warped. Thus, the number of failures is increased due to the warped wafer in later steps, to disadvantageously reduce the yield as a result.
In the aforementioned method of reducing the number of dislocations in the GaN-based semiconductor layer by the dislocation loop effect of the quantum dots, the dislocation density can be reduced to only about 108 cmxe2x88x922.
An object of the present invention is to provide a method of forming a nitride-based semiconductor capable of forming a nitride-based semiconductor layer having low dislocation density with a small thickness.
Another object of the present invention is to provide a nitride-based semiconductor element having excellent element characteristics, including a nitride-based semiconductor layer having low dislocation density with a small thickness.
A method of forming a nitride-based semiconductor according to a first aspect of the present invention comprises steps of laterally growing a nitride-based semiconductor layer on the upper surface of an underlayer and forming quantum dots on the laterally grown nitride-based semiconductor layer.
In the method of forming a nitride-based semiconductor according to the first aspect, the nitride-based semiconductor is laterally grown on the upper surface of the underlayer and the quantum dots are formed on the laterally grown nitride-based semiconductor layer, whereby the number of dislocations reduced by the lateral growth can be further reduced by a dislocation loop effect by the quantum dots. Thus, a nitride-based semiconductor layer having lower dislocation density can be formed as compared with a case of reducing the number of dislocations only by lateral growth. Consequently, a high-quality nitride-based semiconductor having a small number of dislocations can be formed. The number of dislocations can be sufficiently reduced by single lateral growth due to the effects of reducing the number of dislocations by the lateral growth and the quantum dots, whereby the lateral growth may not be repeated for attaining a sufficient effect of reducing the number of dislocations. Thus, the thickness of the nitride-based semiconductor layer can be reduced as compared with the case of repeating lateral growth, whereby the degree of bowing of the wafer can be reduced. Consequently, the number of failures resulting from bowing of the wafer can be reduced in later steps, whereby the yield can be improved.
The aforementioned method of forming a nitride-based semiconductor according to the first aspect preferably further comprises a step of forming mask layers on the upper surface of the underlayer for partially exposing the upper surface of the underlayer, the step of laterally growing the nitride-based semiconductor layer preferably includes a step of epitaxially laterally overgrowing the nitride-based semiconductor layer on the upper surface of the underlayer partially exposed between the mask layers thereby forming facets consisting of the nitride-based semiconductor layer, and the step of forming the quantum dots preferably includes a step of forming the quantum dots on the surfaces of the facets consisting of the nitride-based semiconductor layer. According to this structure, the number of dislocations partially bent by the facets can be reduced by the dislocation loop effect of the quantum dots, whereby no large facets may be formed for reducing the number of dislocations. Thus, the number of dislocations can be sufficiently reduced with a smaller thickness, whereby the degree of bowing of the wafer can be further reduced. Consequently, the number of failures resulting from bowing of the wafer can be further reduced in the later steps, whereby the yield can be improved. In addition, the number of dislocations remaining on the tops of the facets can be reduced due to the dislocation loop effect of the quantum dots, whereby the number of dislocations present above the centers of openings between the mask layers can be reduced. In this case, the facets may include facets having a triangular section. Alternatively, the facets may include facets having a trapezoidal section.
In the aforementioned structure, the mask layers preferably contain at least one material selected from a group consisting of dielectrics such as SiO2 and SiN, high melting point metals such as W having melting points of at least 1200xc2x0 C. and alloys of the high melting point metals. According to this structure, the nitride-based semiconductor layer can be readily epitaxially laterally overgrown through the mask layers.
The aforementioned method of forming a nitride-based semiconductor according to the first aspect preferably further comprises a step of forming mask layers on the upper surface of the underlayer for partially exposing the upper surface of the underlayer, the step of laterally growing the nitride-based semiconductor layer preferably includes a step of epitaxially laterally overgrowing the nitride-based semiconductor layer on the upper surface of the underlayer partially exposed between the mask layers and on the mask layers thereby forming the nitride-based semiconductor layer having a substantially flat upper surface, and the step of forming the quantum dots preferably includes a step of forming the quantum dots above at least central portions of the mask layers on the substantially flat upper surface of the nitride-based semiconductor layer and above central portions between the mask layers. According to this structure, the number of dislocations present above the central portions of the mask layers and above the central portions of openings between the mask layers resulting from epitaxial lateral overgrowth of the nitride-based semiconductor layer can be reduced by dislocation loops of the quantum dots. Thus, the number of dislocations can be reduced on the overall surface of the wafer by single epitaxial lateral overgrowth without repeating the epitaxial lateral overgrowth. Therefore, the number of dislocations can be sufficiently reduced with a small thickness, whereby the degree of bowing of the wafer can be reduced. Consequently, the number of failures resulting from bowing of the wafer can be reduced in later steps, whereby the yield can be improved.
In this case, the step of forming the quantum dots may include a step of forming the quantum dots only above the central portions of the mask layers on the substantially flat upper surface of the nitride-based semiconductor layer and above the central portions between the mask layers. Further, the mask layers preferably contain at least one material selected from a group consisting of dielectrics such as SiO2 and SiN, high melting point metals such as W having melting points of 1200xc2x0 C. and alloys of the high melting point metals. According to this structure, the nitride-based semiconductor layer can be readily epitaxially laterally overgrown through the mask layers.
In the aforementioned method of forming a nitride-based semiconductor according to the first aspect, the underlayer may include an underlayer consisting of a nitride-based semiconductor formed on a substrate. Further, the quantum dots may contain a nitride-based semiconductor.
In the aforementioned method of forming a nitride-based semiconductor according to the first aspect, the step of forming the quantum dots preferably includes a step of introducing gas containing Si in advance of formation of the quantum dots thereby pretreating the surface of the nitride-based semiconductor layer. According to this structure, the quantum dots can be readily formed on the surface of the nitride-based semiconductor layer.
The aforementioned method of forming a nitride-based semiconductor according to the first aspect preferably further comprises a step of growing a nitride-based semiconductor element layer having an element region on the nitride-based semiconductor layer. According to this structure, the nitride-based semiconductor element layer having an element region is formed on an underlayer defined by the nitride-based semiconductor layer excellently reduced in number of dislocations with a small thickness, whereby a nitride-based semiconductor element having excellent element characteristics can be readily formed.
A method of forming a nitride-based semiconductor according to a second aspect of the present invention comprises steps of forming mask layers on the upper surface of an underlayer for partially exposing the upper surface of the underlayer, forming quantum dots on the upper surface of the underlayer partially exposed between the mask layers and laterally growing a nitride-based semiconductor layer on the quantum dots formed on the partially exposed upper surface of the underlayer.
In the method of forming a nitride-based semiconductor according to the second aspect, the quantum dots are formed on the upper surface of the underlayer partially exposed between the mask layers defining a lateral growth interface for the nitride-based semiconductor layer in advance of lateral growth of the nitride-based semiconductor layer as hereinabove described, whereby the number of dislocations reduced by the dislocation loop effect of the quantum dots can be further reduced by the subsequent lateral growth. Thus, a nitride-based semiconductor layer having a smaller number of dislocations can be formed as compared with a case of reducing the number of dislocations only by lateral growth. Consequently, a high-quality nitride-based semiconductor having a small number of dislocations can be formed. The number of dislocations can sufficiently reduced by single lateral growth due to the effects of reducing the number of dislocations by the lateral growth and the quantum dots, whereby the lateral growth may not be repeated for attaining a sufficient effect of reducing the number of dislocations. Thus, the thickness of the nitride-based semiconductor layer can be reduced as compared with the case of repeating lateral growth, whereby the degree of bowing of the wafer can be reduced. Consequently, the number of failures resulting from bowing of the wafer can be reduced in later steps, whereby the yield can be improved.
In the aforementioned method of forming a nitride-based semiconductor according to the second aspect, the mask layers preferably contain at least one material selected from a group consisting of dielectrics such as SiO2 and SiN, high melting point metals such as W having melting points of at least 1200xc2x0 C. and alloys of the high melting point metals. According to this structure, the nitride-based semiconductor layer can be readily epitaxially laterally overgrown through the mask layers.
In the aforementioned method of forming a nitride-based semiconductor according to the second aspect, the underlayer may include an underlayer consisting of a nitride-based semiconductor formed on a substrate. Further, the quantum dots may contain a nitride-based semiconductor.
In the aforementioned method of forming a nitride-based semiconductor according to the second aspect, the step of forming the quantum dots preferably includes a step of introducing gas containing Si in advance of formation of the quantum dots thereby pretreating the surface of the nitride-based semiconductor layer. According to this structure, the quantum dots can be readily formed on the surface of the nitride-based semiconductor layer.
The aforementioned method of forming a nitride-based semiconductor according to the second aspect preferably further comprises a step of growing a nitride-based semiconductor element layer having an element region on the nitride-based semiconductor layer. According to this structure, the nitride-based semiconductor element layer having an element region is formed on an underlayer defined by the nitride-based semiconductor layer excellently reduced in number of dislocations with a small thickness, whereby a nitride-based semiconductor element having excellent element characteristics can be readily formed.
A nitride-based semiconductor element according to a third aspect of the present invention comprises a nitride-based semiconductor layer formed on the upper surface of an underlayer by lateral growth, quantum dots formed on the nitride-based semiconductor layer, and a nitride-based semiconductor element layer having an element region formed on the nitride-based semiconductor layer.
The nitride-based semiconductor element according to the third aspect is provided with the laterally grown nitride-based semiconductor layer and the quantum dots formed on the nitride-based semiconductor layer as hereinabove described, whereby the number of dislocations in the nitride-based semiconductor layer reduced by lateral growth can be further reduced by the dislocation loop effect of the quantum dots. Thus, a nitride-based semiconductor having a smaller number of dislocations can be formed as compared with a case of reducing the number of dislocations only by lateral growth. Consequently, a high-quality nitride-based semiconductor having a small number of dislocations can be formed. Further, the number of dislocations can be sufficiently reduced by single lateral growth due to the effects of reducing the number of dislocations by the lateral growth and the quantum dots, whereby the lateral growth may not be repeated for attaining a sufficient effect of reducing the number of dislocations. Thus, the thickness of the nitride-based semiconductor layer can be reduced as compared with the case of repeating the lateral growth, whereby the degree of bowing of the wafer can be reduced. Consequently, the number of failures resulting from bowing of the wafer can be reduced in later steps, whereby the yield can be improved. When the nitride-based semiconductor element layer having an element region is grown on such a high-quality nitride-based semiconductor layer having a small number of dislocations, a nitride-based semiconductor element having excellent element characteristics can be readily obtained.
In the aforementioned nitride-based semiconductor element according to the third aspect, the nitride-based semiconductor layer preferably includes facets consisting of a nitride-based semiconductor layer formed by epitaxial lateral overgrowth, and the quantum dots are preferably formed on the surfaces of the facets consisting of the nitride-based semiconductor layer. According to this structure, the number of dislocations partially bent by the facets can be reduced by a dislocation loop effect of the quantum dots, whereby no large facets may be formed for reducing the number of dislocations. Thus, the number of dislocations can be reduced with a smaller thickness, whereby the degree of bowing of the wafer can be further reduced. Consequently, the yield can be more improved. In addition, dislocations remaining on the tops of the facets can be reduced by the dislocation loop effect of the quantum dots, whereby the number of dislocations present above the centers of openings between the mask layers can be reduced. In this case, the facets may include facets having a triangular section. Alternatively, the facets may include facets having a trapezoidal section.
The aforementioned nitride-based semiconductor element according to the third aspect preferably further comprises mask layers formed on the upper surface of the underlayer for partially exposing the upper surface of the underlayer, the nitride-based semiconductor layer preferably includes a nitride-based semiconductor layer having a substantially flat upper surface formed by epitaxial lateral overgrowth, and the quantum dots are preferably formed above at least central portions of the mask layers on the substantially flat upper surface of the nitride-based semiconductor layer and above central portions between the mask layers. According to this structure, the number of dislocations present above the central portions of the mask layers and above openings of the mask layers resulting from epitaxial lateral overgrowth of the nitride-based semiconductor layer can be reduced due to the dislocation loop effect of the quantum dots, whereby the epitaxial lateral overgrowth may not be repeated but the number of dislocations can be reduced on the overall surface of the wafer through single epitaxial lateral overgrowth. Thus, the number of dislocations can be sufficiently reduced with a small thickness, whereby the degree of bowing of the wafer can be reduced. Consequently, the number of failures resulting from bowing of the wafer can be reduced in later steps, whereby the yield can be improved.
In this case, the quantum dots may be formed only above the central portions of the mask layers on the substantially flat upper surface of the nitride-based semiconductor layer and above the central portions between the mask layers. Further, the mask layers preferably contain at least one material selected from a group consisting of dielectrics such as SiO2 and SiN, high melting point metals such as W having melting points of a least 1200xc2x0 C. and alloys of the high melting point metals. According to this structure, the nitride-based semiconductor layer can be readily epitaxially laterally overgrown through the mask layers.
In the aforementioned nitride-based semiconductor element according to the third aspect, the underlayer may include an underlayer consisting of a nitride-based semiconductor formed on a substrate. Further, the quantum dots may contain a nitride-based semiconductor.
A nitride-based semiconductor element according to a fourth aspect of the present invention comprises mask layers formed on the upper surface of an underlayer for partially exposing the upper surface of the underlayer, quantum dots formed on the upper surface of the underlayer partially exposed between the mask layers, and a nitride-based semiconductor layer formed by lateral growth on the quantum dots formed on the partially exposed upper surface of the underlayer.
In the nitride-based semiconductor device according to the fourth aspect, the quantum dots are formed on the upper surface of the underlayer partially exposed between the mask layers defining a lateral growth interface for the nitride-based semiconductor layer as hereinabove described, whereby the number of dislocations reduced by a dislocation loop effect of the quantum dots can be further reduced by the subsequent lateral growth. Thus, a nitride-based semiconductor having a smaller number of dislocations can be formed as compared with a case of reducing the number of dislocations by only lateral growth. Consequently, a high-quality nitride-based semiconductor having a small number of dislocations can be formed. The number of dislocations can be sufficiently reduced by single lateral growth due to the effects of reducing the number of dislocations by the lateral growth and the quantum dots, whereby the lateral growth may not be repeated for attaining a sufficient effect of reducing the number of dislocations. Thus, the thickness of the nitride-based semiconductor layer can be reduced as compared with the case of repeating the lateral growth, whereby the degree of bowing of the wafer can be reduced. Consequently, the number of failures resulting from bowing of the wafer can be reduced in later steps, whereby the yield can be improved.
In the aforementioned nitride-based semiconductor element according to the fourth aspect, the mask layers preferably contain at least one material selected from a group consisting of dielectrics such as SiO2 and SiN, high melting point metals such as W having melting points of at least 1200xc2x0 C. and alloys of the high melting point metals. According to this structure, the nitride-based semiconductor layer can be readily epitaxially laterally overgrown through the mask layers.
In the aforementioned nitride-based semiconductor element according to the fourth aspect, the underlayer may include an underlayer consisting of a nitride-based semiconductor formed on a substrate. Further, the quantum dots may contain a nitride-based semiconductor.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.